1. Field of the Invention
This invention relates generally to magnetic resonance imaging (MRI) methods and apparatus employing nuclear magnetic resonance (NMR) phenomena. The invention is particularly directed to a novel RF coil (especially useful for transmitting NMR RF nutation pulses into a MR image volume) substantially without obstructing access to the image volume in a transverse magnet type of MRI system.
2. Related Art
MRI systems of many different designs are now commercially available. Some MRI systems utilize solenoidal cryogenic electromagnets to create a uniform static polarizing field B.sub.o to support the NMR phenomenon. Others bracket the MR image volume with a pair of opposed magnet poles. This latter class of MRI systems is sometimes referred to as utilizing a "transverse" magnet structure. As those in the art will realize, such structures have employed permanent magnets and/or electromagnets (which may be either resistive or cryogenic superconducting). The magnetic circuit structure for conducting magnetic flux between the opposing transverse poles of the magnet may take various forms (e.g., "C" shape, "H" or "four-poster" shape). A four-poster type of transverse magnet MRI system is disclosed, for example, in commonly assigned U.S. Pat. No. 4,829,252--Kaufman issued May 9, 1989 and entitled, "MRI SYSTEM WITH OPEN ACCESS TO PATIENT IMAGE VOLUME" (the entire content of which is hereby incorporated herein by reference). One advantage of a transverse magnet system is that it provides relatively unimpeded access to the image volume --at least insofar as the magnet structure itself is concerned.
However, in addition to the static polarizing magnetic field B.sub.o that is required for MRI, there are other magnetic field and RF field requirements as well. As is well known in the art and as is typical in commercially available MRI systems, sets of magnetic gradient coils are employed for quickly switching into effect gradients along mutually orthogonal X,Y,Z coordinates in the background field B.sub.o during selected portions of an MRI data acquisition cycle. In a transverse magnet system, such magnetic gradient coils are typically of a relatively flat pancake-type configuration (e.g., such as those shown in related U.S. Pat. No. 4,829,252--Kaufman, the entire content of which is hereby incorporated by reference).
In order to purposefully nutate NMR nuclei during MRI data acquisition sequences, it is typically necessary at certain times to transmit RF pulses of predetermined frequency spectra, shape, strength, time duration, etc. At other times in the MRI data acquisition cycle, NMR RF response signals are detected from the image volume. These RF responses are subsequently processed to ultimately produce a visual representation of the spatial distribution of NMR nuclei within the object being imaged (e.g., a living human body).
The transmitted NMR RF pulses are typically of relatively high intensity and it is desirable to have such fields as uniformly distributed as possible within the image volume (the RF magnetic field being disposed transverse to the static polarizing magnetic field). One prior art approach has utilized Helmholtz coils in an apertured coil former so as to nevertheless provide some access to the image volume (e.g., through the coil former apertures) even when in place. One such example is the RF transmit coil 601 as depicted in FIGS. 6 and 7 of the related U.S. Pat. No. 4,829,252--Kaufman and as also indicated at reference numeral 100 in FIG. 1 of the present application.
Another prior art RF coil design providing reduced obstruction access to the image volume in a transverse magnet MRI system is found at U.S. Pat. No. 4,968,937--Akgun. Akgun does away with Helmholtz coils or solenoids or the like in favor of opposing current "sheets" defined by RF currents passing along sets of linear conductors disposed above and below the image volume. Although Akgun discloses embodiments wherein some of the linear current paths in each set have varying inter-path spacings, varying numbers of conductors and/or varying conductor dimensions, all of the Akgun current sheets utilize a plurality of conductors electrically connected in parallel with a separate return conductor structure being disposed on another structure fastened to the "back" side of each current sheet structure.
Akgun also suggests that if the return current paths are shielded (e.g., so as to effectively become shielded coaxial RF transmission lines), then they may be "positioned anywhere convenient instead of behind the current sheet" (column 6, lines 7-11). Such a composite and/or shielded structure for the return current path is believed to a major disadvantage of the Akgun approach (e.g., introducing complexity, cost, manufacturing difficulties, etc.). In addition, since the current sheet conductors are all connected in parallel, it may be difficult always to precisely control the distribution of RF currents between the various conductors of the current sheet.